In the process of vascular surgery it frequently becomes necessary to replace or bypass a defective vein or artery or portions thereof. A wide variety of materials have been examined and tested as grafts. Examples are bovine and porcine blood vessels, and tubular prosthesis woven or knitted to Teflon and other synthetic fibers.
Blood vessels taken from umbilical cords have recently been disclosed as being highly effective as grafts as well as for other prosthetic applications in that they appear to be completely free of thrombogenicity and antigenicity. However, the umbilical cord vessels are of relatively small diameter, so that tubular prosthesis of these vessels have a maximum internal diameter of about 5 mm. For larger vessels such as the aorta and the aorta in combination with the two iliac arteries, knitted tubular prostheses are best from the standpoint of tissue ingrowth and freedom from thrombogenicity. Furthermore, polyester fiber, and, in particular, polyester fiber manufactured by Dupont and sold under the tradename of Dacron (polyethylene terephthalate appears to be the best fiber.
A number of problems arise in connection with the knitted tubular prostheses. The first problem is that of leakage of blood through the wall of the prosthesis. This problem is met by making the openings in the knitted wall as small as possible and by preclotting the blood of the patient in the wall of the prosthesis. Optimally, all openings in the wall of the prosthesis are closed with clotted blood prior to joining the prosthesis to the vessels of the patient by anastomosis.
In order that the graft should be essentially circular in section, thereby providing for minimum resistance to flow of blood therethrough, it is necessary that the prosthesis be crimped. Sprial crimping which provides an appropriate degree of rigidity to the tubular prosthesis has been disclosed in U.S. Pat. No. 3,945,052 issued Mar. 23, 1976. Circular crimping which provides additional protection against twisting is taught in application Ser. No. 638,580 referred to above.
Once the graft is inserted and the patient is closed up, success of the graft depends upon ingrowth of tissue through the wall of the prosthesis to form an interior which will resemble the vessels into which the graft has been inserted insofar as it serves to provide a lumen for the flow of blood therethrough without causing either thrombosis or rejection.
It has been found that the ingrowth of tissue is facilitated by "roughening" both the interior and the exterior of the tubular prosthesis. Sauvage in German Offenlegungsschrift No. 24 61 370 has disclosed a prosthesis which is circularly knitted and which has a pile velour both interior and exterior to the wall of the prosthesis. Further, the prosthesis crimped, though in an irregular fashion, so that the rigidity of the prosthesis varies along its length.
Sauvage's prosthesis is knitted of at least three threads and preferably four, all of the threads forming portions of the wall, one thread, periodically, being drawn into exterior loops and another thread, also periodically, being drawn into interior loops. As is evident, where one or two threads are used exclusively to form the wall of the prosthesis and the other two threads are used, at least in part, for forming the wall of the prosthesis, the wall of the prosthesis is necessarily relatively thick. Further, the number of loops, both interior and exterior per unit area cannot be maximal since the looped threads are also used for forming the wall of the prosthesis. Since the loops have been found to facilitate the growth of tissue in the prosthesis wall, it is evident that it would be desirable that the number of loops per unit area of wall be as large as possible.
As can be seen, then, it would be desirable that a number of improvements be introduced into the construction of knitted fabric prosthesis, these improvements including smaller openings between the threads, greater number of loops per unit area, thinner fabric taking into account both the wall thickness and the height of the interior and exterior loops, the reduction in thickness to be achieved without increase in porosity or loss of rigidity, and more rapid ingrowth of tissue.